Pillow Block Bearings Information
Pillow block bearings, sometimes referred to as plummer block bearings, are mounted bearings that are used to provide load support for a rotating shaft. Their mounting surface is on a plane parallel and offset to the axis of the shaft. Mounting holes or slots in the base or feet of pillow block bearings permit adjustment and easy mounting.
Typical pillow block bearing construction. Image Credit: Sup Bearing
Pillow block bearings are constructed of a housing and a bearing. Many pillow blocks have a replaceable bearing, referred to as an insert bearing. Bearings used with pillow block housings usually are self- aligning to compensate for angular misalignments. Pillow blocks are available with many different bearing types, including:
Plain bearings are used to constrain, guide, or reduce friction in linear applications. They use a sliding action instead of the rolling action used by ball, roller, and needle bearings. Plain bearings can be constructed of a variety of materials, but are typically bronze, a graphite-metal allow, or plastic. Metal plain pillow block bearings must be lubricated properly to reduce wear and friction. Bronze plain bearings may also be either impregnated with oil or self-lubricating.
Ball bearings have a ball as the rolling element. They are used to provide smooth, low friction motion in rotary applications. Construction consists of an inner and outer ring, balls, and usually a cage or ball separator. Ball pillow block bearings feature an inner and outer ring and balls. They may be equipped with a cage or ball separator.
Roller bearings have a roller as the rolling element. They are used to provide smooth, low friction motion in rotary applications. Construction consists of an inner and outer ring, rollers, and usually a cage or roller separator. There are three principal types of roller bearings used in pillow blocks:
Tapered-roller bearings use conical rollers that run on conical races. They can support both radial and axial loads as well as carry higher loads than ball bearings due to having greater contact area. Tapered-roller bearings contain an inner ring (cone), an outer ring (cup), a cage, and rollers. They have high radial and axial load capacities when used at low to intermediate speed levels. The thrust level of tapered-roller pillow block bearings is about 60% of the radial capacity. They are usually more expensive.
Spherical- roller bearings are self-aligning, double-row, combination radial and thrust bearings. The rolling element in these pillow block bearings has a crowned or spherical shape. Spherical-roller pillow block bearings are superior when dealing with high loads and loads that require tolerance to shock; however, they have limited speed capabilities.
Cylindrical-roller bearings are bearings with high radial capacities and moderate thrust loads. They can be crowned or end-relieved for reduced-stress concentrations. Cylindrical-roller pillow block bearings produce low friction and allow for high-speed.
Needle-roller bearings have a needle roller as the rolling element. They are similar to cylindrical roller bearings but have a smaller diameter-to-length ratio. By controlling the circumferential clearance between rollers, or needles, rolling elements are kept parallel to the shaft axis. Needle roller bearings are designed for radial load applications where a low profile is desired.
Hydrodynamic bearings are fluid film bearings that rely on a film of oil or air to create a clearance between the moving and stationary elements. Hydrodynamic pillow block bearings maintain high stiffness, load capacity, and long bearing life, but are considerably more complex and expensive than standard rolling element pillow block bearings.
Bearing Housing Variations
Plummer block housings are reinforced for heavy-duty applications, such as those found in manufacturing, mining, and construction industries.
Split bearing housings are completely split to the shaft for easy bearing replacement and inspection.
Spilt housing pillow block. Image Credit: Miether Bearing Products
With pillow block bearings, product selection is based on:
- Shaft diameter- the diameter must be matched with the appropriate bore dimensions of the bearing unit. Properly-sized pillow block bearings easily slide onto the shaft and into position, ready to be secured by the locking device.
- Shaft height- the shaft height or backing height is determined by measuring the distance from the base of a pillow block bearing to the center of the shaft.
- Shaft size- The maximum diameter of the shaft or bore size of the mounted bearing.
- Maximum speed- The maximum allowable shaft speed that the bearing has been designed to operate with.
- Maximum load- The dynamic load capacity for bearings capable of combination radial and thrust loads or the maximum permissible radial load for bearings that can take radial loads only. Load rating varies as a function of shaft speed. Consult the manufacturer for load rating at various shaft speeds.
Bearing housing protects the bearings during operation as well as providing a mechanism for the bearing to be secured while in use. Pillow block housing can be made from a variety of materials with different properties.
Aluminum has a high load-carrying capacity, fatigue strength, and thermal conductivity.
Cast iron refers to a family of materials whose major constituent is iron, with important trace amounts of carbon and silicon. Cast irons are natural composite materials whose properties are determined by their microstructures - the stable and metastable phases formed during solidification or subsequent heat treatment. The major microstructural constituents of cast irons are: the chemical and morphological forms taken by carbon, and the continuous metal matrix in which the carbon and/or carbide are dispersed. Cast iron is the most common material for pillow block bearing housings.
Pressed steel is a low carbon steel, which has been pressed rather than machined. Pressed steel pillow blocks are less expensive than cast or machined pillow blocks are used for lower load- carrying applications.
Plastic refers to numerous organic, synthetic, or processed materials that are mostly thermoplastic or thermosetting polymers of high molecular weight and that can be made into objects, films, or filaments. Common plastic materials include Acetal, Nylon / Polyamide and PTFE / Teflon.
Nylon, comprising several grades of polyamides, is a general-purpose material in wide use; it is tough and resistant and has good pressure ratings.
PTFE (polytetrafluoroethylene) is an insoluble compound that exhibits a high degree of chemical resistance and a low coefficient of friction. It is sometimes marketed in proprietary classes of materials such as Teflon®, a registered trademark of DuPont Dow Elastomers.
Acetal polymers are semi-crystalline. They offer excellent inherent lubricity, fatigue resistance, and chemical resistance. Acetal suffers from out-gassing problems at elevated temperatures, and is brittle at low temperatures. Glass filled, and added lubrication grades are available, flame-retardant grades are not. Brand names include Celcon® (Hoechst Celanese), Delrin® (Dupont), Thermocomp® (LNP), Ultraform® (BASF), and Acetron® (DSM Polymers).
Stainless steel is chemical and corrosion resistant and can have relatively high-pressure ratings
The material the bearing is made of has a significant effect on the performance of the bearing unit.
Steel is a commercial iron that contains carbon in any amount up to about 1.7 percent as an essential alloying constituent. It is malleable when under suitable conditions, and is distinguished from cast iron by its malleability and lower carbon content. Bearings are usually made of high carbon steel such as 52100.
Stainless steel is chemical and corrosion resistant and can have relatively high pressure ratings.
Sintered metals are often called powdered-metal bearings. They are self-lubricating, simple, and low in cost. These bronze and copper alloys are porous materials that can be impregnated with oil, graphite, or PTFE. They are not suitable for heavily loaded applications, but are useful where lubrication is inconvenient.
Graphite-metal alloy has a bearing surface of an alloy manufactured from molten graphite and metal. This material is self-lubricating.
Plastic refers to any of numerous organic, synthetic, or processed materials that are mostly thermoplastic or thermosetting polymers of high molecular weight and that can be made into objects, films, or filaments. They do not require lubrication and their high strength lends to a variety of applications. Common plastic materials include Acetal, Nylon / Polyamide and PTFE / Teflon.
Shaft Securing Method
Pillow block insert bearings with setscrew and locking collar.
Setscrew bearings have setscrews for securing the inner ring to a shaft.
Locking collar bearings have a locknut for securing the inner ring to a shaft.
Some bearings have no securing methods for securing to a shaft
As in all bearings, proper lubrication is required to reduce wear and friction. Commercial lubricants are grouped into three generic types: greases, oils and solid films. Lubrication choices include manual lubrication and self-lubricating.
Manual lubrication requires a pump or a fitting mounted on the housing to deliver lubrication to the friction points of the bearing.
Typical pillow block bearing lubrication. FYH
Self- lubricating bearings are constructed of a material such as oil-impregnated bronze, or graphite / metal alloy that do not require external lubrication.
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